I've had a lot of luck importing used books (which I only do if it's out of print, the new prices are already fairly affordable even for tankobon) from Buyee, which I only leaned recently has a feature on their site that will search multiple stores. (They do charge separate fees if you're ordering from multiple stories, then you can have them bundle that into a single package to send to you.)
Since I'm mostly doing this for older stuff by authors I'm catching up on, I'll add that early volumes are often only 100 yen, but last volumes can be double or triple the cover price. There just weren't as many copies printed in the first place, and since the series ended, no reprints; at least one took several tries to even find a copy that hadn't been sold out from under the listing. (I assume, since the seller cancelled after the order was placed.)
